It has been known for many years that nitrides of silicon have properties different from silicon dioxide and that some of these properties might be advantageous in certain applications. Silicon nitride and silicon oxynitrides can be produced in various ways as by reaction of silicon and/or silicon dioxide with ammonia, and products of this type would have utility for some special applications.
However, there are many reasons why the commercial use of such products has been very limited, why research relating to nitrided silicon products has not been extensive, and why large capital investment for research and development in this area did not appear to be justified. It is difficult and expensive to produce silicon nitride products or silicon oxynitride products. Silicon dioxide (silica) does not react readily with nitrogen, although it is possible with appropriate reaction conditions to produce oxynitrides by reacting particles of silica with anhydrous ammonia.
In the field of microelectronics, scientists have given some consideration to possible uses of silicon oxynitride films because of the unique dielectric properties and other properties. Such films can be produced by chemical vapor deposition or by nitridation of silicon surfaces or thin silicon-dioxide films. Thin silica films made by a sol-gel process can be penetrated by ammonia, perhaps because of the microporosity and cracking of the dried film. At a temperature of 1000.degree. C. to 1200.degree. C., anhydrous ammonia can react with the silica film to produce oxynitrides with special properties.
Consideration has also been given to the manufacture of glass or glass-ceramic products from compositions containing silica (SiO.sub.2) and nitrogen (N) as base components as described in Corning U.S. Pat. No. 4,222,760. However, that patent points out that the practical glass-forming region is quite small in the simple ternary SiO.sub.2 --Al.sub.2 O.sub.3 --N system (FIG. 8) and is essentially non-existent in the simple binary SiO.sub.2 --N system.
Silicon oxynitride glasses can be produced by melting a mixture of oxide and nitride powders at a high temperature, such as 1600.degree. C. to 1700.degree. C. or more. Oxides of aluminum and other metals may be used (i.e., Ca, Li, Mg or Y). The nitrogen source may be Si.sub.3 N.sub.4 or AlN, for example. The oxynitride glass is potentially useful in making special plate glass or glass fibers (See U.S. Pat. No. 4,609,631).
Oxynitrides have some desirable properties which may be superior to those of quartz glass and may have potential value in the semiconductor industry. However, it appears that such potential, if any, has yet to be realized and that the use of oxynitride glass in connection with the commercial manufacture and processing of silicon-wafers and other semiconductor devices has not been found worthwhile.
To date there has been no practical substitute for quartz glass in the commercial manufacture of silicon semiconductors. The modern glass crucibles used in Czochralski (Cz) crystal-growing furnaces have been formed of silica having a very high purity (i.e., a purity of at least 99.99 percent). Substantial amounts of nitrogen cannot be tolerated in Cz crucibles. For more than two decades the manufacturers of silicon crystal have insisted that the crucibles used in crystal-growing furnaces be transparent and free of significant amounts of nitrogen or cristobalite.
Because of the importance of microelectronics and computers, there is a high demand for ultra-pure silica glass in the manufacture of modern micro-chips. The semiconductor industry is becoming increasingly intolerant with respect to contaminants in quartz glass. In order to meet modern requirements for the processing of semiconductor wafers, a glass should contain at least 99.995 percent by weight of silica. The ultra-pure synthetic fused quartz commonly used for this purpose usually has a purity of about 99.999 percent.
Prior to the present invention, the presence of significant amounts of chemically-bound nitrogen in a quartz glass used in semiconductor manufacture would have been considered highly undesirable. Nitrogen heretofore appeared to be an impurity to be avoided.
The percentage of the nitrogen impurity in a commercial quartz glass is low but is not often measured or reported because of the difficulty of ascertaining the nitrogen content with reasonable accuracy. The analytical detection problem is another good reason why the unusual properties and advantages of chemically-bound nitrogen were heretofore not understood nor appreciated in the glass industry.
For several decades vitreous silica products essentially free of crystalline silica have been used extensively because of exceptional thermal shock resistance and other advantageous physical properties. However, these products have a limited useful life when heated above 1200.degree. C. and other disadvantages because of limited resistance to deformation, the devitrification of the glass, and the damage resulting from the crystallographic alpha-beta inversion during heating and cooling of the devitrified glass. There has been a need for a practical solution to these problems for several decades, particularly the devitrification problem, but no simple solution was found prior to the present invention.
There has also been a need to remedy other deficiencies in certain products and processes involving the use of quartz glass or vitreous silica. For example, serious problems have been encountered when attempting to cast elemental silicon in silica molds, making it necessary to tolerate the expense and inefficiency of temporary breakaway casting molds.
In the semiconductor industry, modern epitaxy reactors, diffusion furnaces, CVD equipment and other high-temperature equipment have a great need for effective thermal radiation heat shields. There have been some attempts to meet this need, but they have been crude and generally unsatisfactory.